Hello, I'm Keith and happy to help you with your question. You are talking to a retired RAPC and AGC(SPS) Major.
Your military pension is certainly subject to UK taxation and you cannot get around that. Your accountant is simply wrong on this matter. All UK public service pensions (ie Government, Quasi-government and many Local Authority) are taxable in the UK. He should know this, but may not. I would advise you to draw his attention to this simple UK taxation principle.. State pensions, although taxable emoluments in the UK, are paid gross as you are probably well aware.
Moving on the Belgian tax Expatica.com has the following dismal news:
'Expatriates and Belgian citizens alike suffer from one of the highest taxation rates in the EU. It amounts to an effective tax rate (including social security) of well over 50 percent for the highest earners. This compares to an average 45 percent in Europe. An expatriate working in Belgium will typically be liable for Belgian income tax. Additionally, property tax, gift and inheritance tax may be relevant. In most circumstances there are no capital gains taxes or wealth tax for individuals in Belgium, thus pushing the tax burden firmly onto the employee.'
There is a special expatriate tax status which apples to income from professional duties carried out in the country, but you may not qualify for this.
The tax you pay on your UK income is, under the Double Taxation Treaty between the UK and Belgium, allowable as a tax credit against any Belgian Tax liability on the same source of income. In view of the very high tax rates, about 50%, I fear your pensions may attract additional tax in Belgium. I see you retain a local taxation professional to assist you in this matter, very wise.
I am so sorry to have to convey such dismal news. Always bear in mind Benjamin Franklin's dictum that in life there are but two certainties, death and taxes. I fear you have caught a double whammy in this matter.
Small point, in view of the almost certain necessity of paying some Belgian tax on top of your UK you could consider asking HMRC to apply a tax code of 'BR' to all your pensions which will tax everything but your State Pension at 20%. Your UK personal allowance of 10K is of no use to you if it reduces your UK tax merely for the Belgian authorities to collect more. Check your position viz a viz Belgian Income Tax before you do get your codes adjusted.